Petrology of the bimodal Cenozoic volcanism of the Kapsiki plateau (northernmost Cameroon, Central Africa)

Abstract

The Kapsiki Plateau is the northernmost volcanic zone of the Cameroon Line. The volcanism (27–35Ma) is of alkaline type and has a typical bimodal lava series diversity with basalts and hawaiite as mafic lavas and phonolites, trachytes, and rhyolites as felsic lavas. Mg-rich olivine phenocrysts occur only in basalts. The hawaiite contains andesine, olivine and Ca-rich pyroxene phenocrysts, and sanidine and quartz xenocrysts. The phonolites contain alkali–feldspar and Na-rich clinopyroxene phenocrysts. Two types of trachytes occur: peralkaline trachytes, with Ti-, Na- and F-rich aegirine augite, richterite, arfvedsonite phenocrysts and non-peralkaline trachytes, with an aenigmatite-type undetermined mineral, Ti-rich biotite and zircon phenocrysts. Similarly, two types of rhyolites occur: peralkaline with quartz and arfvedsonite phenocrysts and non-peralkaline with quartz and biotite phenocrysts.Differentiation indices (Thornton and Tuttle, 1960) of the lavas range from 22 to 97 with a large gap in the range 34–82. Some basalts are primitive (530ppm Ni, 1100ppm Cr). In basaltic lavas, phonolites and non-peralkaline trachytes and rhyolites, Zr and Nb covary with approximately constant ratios (3.1<Zr/Nb<5.3). However, peralkaline trachytes and rhyolites have high concentrations in Zr (up to 2180ppm) and Nb (up to 780ppm), with correlative higher Zr/Nb ratios (6.3–8.3). Some rhyolites have abnormal REE patterns (with kinks), depleted in light-REE, probably resulting from stability of Na–REE–F complexes under hydrothermal conditions. Despite a large gap between basaltic and felsic lavas, major- and trace-element distributions indicate co-magmatism for both the basaltic and felsic lavas. The differentiation of the lava series is dominated by crystal fractionation, the role of fluids in rhyolite genesis being of minor effect, as evidenced by constant values of Y/Ho and Zr/Hf throughout the series.The Kapsiki Plateau basalts are similar in their chemical and isotopic data character to other basalts from both the continental and oceanic sectors of the Cameroon Line. The continental crust appears to have no significant role in their genesis. The hawaiite mineralogical and geochemical characteristics are consistent with an origin by mixing of basaltic and felsic (phonolitic) magmas. The Kapsiki Plateau basaltic magmas may have originated from an infra-asthenospheric reservoir similarly to other basaltic magmas generated throughout the Cameroon Line. The Sr-isotope variations observed in trachytes and rhyolites point to some contamination of the magmas by crustal materials, while the Nd isotopic composition is only slightly affected.